Modern society tends to promote sedentary lifestyles, in which office workers and others must perform their daily work while seated. Long periods of sitting are increasingly being identified as a major cause of lower back pain. Studies directed to that phenomenon have concluded that changes in the curvature of the spine that occur when a person sits down and remains seated for long lengths of time, are one cause of lower back pain. One such study is entitled Ergonomics in the Design of Office Furniture, by Kroemer, Ing, and Robinette, and published in Industrial Medicine, Vol. 38, No. 4 (April, 1969). Additional studies have been conducted to determine the extent of changes to spinal curvature that result from moving from a standing position to a seated position, and the effect upon the spine of seating posture and angle of recline when seated. Two typical studies are Posture of the Trunk When Sitting on Forward Reclining Seats, by Bendix and Biering-Sxc3x8rensen, published in Scandinavian Journal of Rehabilitative Medicine, Vol 15 (1983); and The Influence of Different Sitting Positions on Cervical and Lumbar Posture, by Black, McClure and Polansky, published in SPINE, Vol. 21, No. 1 (1996).
Lumbar lordosis is the term commonly used to refer to the forward curve of the lumbar spine. In one study (Lumbar Lordosisxe2x80x94Effects of Sitting and Standing, by Lord, Small, Dinsay and Watkins, published in SPINE, Vol. 22, No. 21 (1997)), lordosis was measured as an angle between various identifiable points within the spine, with xe2x80x9ctotal lordosisxe2x80x9d being measured from the cephalad endplate of L1 to the cephalad endplate of S1. Other measurements from L2 and S1, from L4 to S1, and from L5 to S1 were also taken to determine the amount of curvature at each of the L1 locations, and the difference in curvature from standing to sitting. The results from that study are that lordosis increases by almost 50 percent when the patient moves from a sitting to a standing position, representing an angular change in total lordosis from 34xc2x115 degrees when sitting to 49xc2x115 degrees when standing. Since lower back pain and increased intradiscal pressure are associated with sitting over long periods of time, there has been an interest in alleviating or mitigating the problem by increasing lordosis when a subject is seated. Various attempts to do this have been proposed, and have led to ergonomically designed chairs having backs that provide lower back support for the lumbar spine. Another proposal has been that seats should be sloped forward to increase the forward tilt of the pelvis when seated. The Kroemer study, however, indicates that, while tilting the seat may increase forward pelvic tilt, also increasing lordosis, it also causes the body to slide forward. Such forward thrust must then be counterbalanced by action of the leg muscles, which ultimately becomes uncomfortable and fatiguing.
Other problems associated with sitting for long periods of time are documented in Sitting Posture and Prevention of Pressure Ulcers, by Defloor and Grypdonck, published in Applied Nursing Research, Vol. 12, No. 3 (August, 1999), Shear vs Pressure as Causative Factors in Skin Blood Flow Occlusion by Bennett, Kavner, Lee and Trainor, published in Arch Physical Medical Rehabilitation, Vol 60 (July, 1979), and Hemorrhoids by Nussain, published in Primary Care, Vol. 26, No. 1 (March, 1999), and include pressure ulcers, decreased blood flow, and aggravation of pre-existing hemorrhoids.
A different, but related problem involving low back pain is frequently experienced by pregnant women. Here, however, the pain may be related to increased lordosis, which can be the result of changes in the center of gravity caused by the added weight of the growing baby in a location that is forward of the body""s normal center of gravity. The added weight requires the mother to adjust her posture to balance and compensate for the added weight. Such adjustment affects both the standing posture and the seated posture. In addition, studies such as Venous Dynamicsxe2x80x94Vericosities, by Sumner, published in Clinical Obstetrics and Gynecology, Vol. 24, No. 3 (September 1981) have determined that two-thirds of women in and after their 29th week of pregnancy experience compression of the right iliac artery at the most prominent point of lumbar lordosis. Potential adverse effects resulting from this blood flow restriction, and on related restricted venous outflow, include interference with fetoplacental nutrition, which is nutrition pertinent to the fetus and its placenta, and decreased blood return to the mother""s heart. Traditional methods of treating these conditions have used physical therapy. However, it does not appear that attempts to alleviate these conditions through the use of an ergonomic seat cushion designed to decrease lordosis in pregnant women, and to improve arterial and venous blood flow in the iliac and pelvic areas, have heretofore been made. The pain and symptoms associated with these conditions may be alleviated by one or more embodiments of the present invention.
A pillow for alleviating pain associated with hemorrhoids and other tenderness in the perineum area has been patented by the inventor in U.S. Pat. No. 6,018,831 to Loomos, and consists of a pillow having foam layers of varying density on either side of a center cavity. The topmost layer extends partially over the cavity on either side, creating a cantilever effect that reduces the pressure gradient upon the body at the transition line separating the foam support layers from the central cavity. This invention, however, does not address the problems of reduced lordosis while seated, nor of occluded blood flow to the skin that results from pressure applied to the skin of the buttocks and perineum areas when seated. While the cantilevered topmost foam layer taught by Loomos does tend to modify the pressure gradient that exists between the foam layered portion of the pillow and the central cavity, the problem is not totally alleviated because the cantilevered design creates a narrow, well-defined perimeter of shear force at the margin between the central cavity and the surrounding foam support layers. This shear force causes a high pressure gradient that tends to occlude skin blood flow into and across skin areas adjacent to that perimeter.
In one embodiment, the ergonomic seating cushion of this invention combines a forward tilt, which tends to increase lordosis, with a dimpled upper surface and a cavity extending through the upper surface. As used in this disclosure, a xe2x80x9cdimpled surfacexe2x80x9d shall refer to a three dimensional surface having elevations and depressions. The interior margins of the cavity below the upper surface are in the shape of an oscillating waveform. The dimpled upper surface and waveform margins reduce large pressure gradients and promote satisfactory skin blood flow to the buttocks, perineum area, and legs. By using foam layers of varying firmness and thickness, an ideal combination of comfort and therapeutic effect may be achieved.
In another embodiment, the cushion is fashioned to create a backward tilt which is designed to tilt the pelvis rearward. Such a design assists pregnant women to overcome the increased lordosis that is caused by a growing baby. The internal configuration of the cushion is similar to that of a forward-tilting cushion, and will decrease the incidence of compression of the right iliac artery and of decreased venous outflow, described above. In this design, other features of the invention have the same beneficial effects described below, yet the tilt to the rear provides additional support for the spine when sitting that largely compensates for changes in the mother""s center of gravity.
Because individuals have different degrees of natural lordosis, and will tolerate different amounts of tilt before experiencing discomfort, the cushion of this invention is intended to be custom made or mass produced; and when custom made, may be given any amount of forward or backward tilt, including no tilt at all, as desired by the intended user.
The upper surface of the cushion of this invention is dimpled to create variances in the pressure gradients experienced by skin immediately adjacent to the cushion surface. Dimples in a foam layer having trough to crest heights of 1 to 2 cm, and that are spaced apart between 2 to 4 cm, create an ideal surface for limiting the amount of skin within a local area that could experience an occluded blood flow. On such a surface, very minor readjustments of seating position will produce large changes in pressure gradients, thereby ensuring that blood flow will reach all areas subject to surface pressures induced by sitting.
As taught by Loomos, a central cavity is provided to eliminate the pressure of sitting upon the perineum area, thereby minimizing or removing the pain and discomfort associated with hemorrhoids and other conditions. In the present invention, the perimeter of the cavity in which shear forces tend to create high pressure gradients is cut to form a series of oscillating waves, which have the effect of varying the margin and minimizing the pressure gradient experienced by the skin adjacent the cavity margin. By minimizing the pressure gradient, lower peak surface pressures are created, and skin blood flow is improved.
The cushion may be tilted forward slightly, to tilt the pelvis and assist in attaining a larger lordosis than would otherwise be present. A forward tilt of between 3 degrees and 10 degrees should be most effective, although actual tilt may be either lesser or greater, and will ultimately be determined by personal comfort and preference. The cushion is normally covered by a fabric, and a fabric should be chosen that exhibits high static friction on both sides. The combination of a dimpled upper surface and high static friction fabric will assist in maintaining the proper posture without sliding forward off the cushion. In the preferred embodiment, three foam layers are used, although embodiments having only two layers are also disclosed. The topmost layer will have an ovoid or elongated cavity, and will be immediately superjacent a middle foam layer having a cavity that is slightly larger than the cavity in the topmost layer. The cavity in the middle layer will have oscillating waveform margins, and the foam will normally be somewhat firmer than that used for the top layer. Also in the preferred embodiment, the middle layer will rest atop a bottom layer. The bottom layer has no cavity, and is intended to provide strength and structure to the cushion. In order to form a tilt, any one or more of the layers may be formed in the shape of a wedge. In the preferred embodiment, the middle layer is shaped like a wedge with an increasing thickness from front to back, the angle of the wedge becoming the angle of tilt.